1. Field of the Invention
This invention relates to a package for an optical element adapted to package or receive an optical element such as a light emitting diode or photodiode.
2. Description of the Prior Art
The principal technical requirements called for in a package for receiving an optical element such as a light emitting diode or photodiode are: (1) that the package has a transparent light transmitting construction through which light is taken into or out of the optical element, (2) that electrodes can be led out of the optical element by providing wiring on the optical element by die-bonding or wire-bonding, and (3) that the package has a hermetically sealed construction in order to preserve its properties for guarding against environmental changes and its reliability. Examples of such packages in the prior art will be described hereinbelow in connection with a photodiode.
FIGS. 1a and 1b show examples of a first type of prior art package for an optical element. In this first type of package, conventional TO-18 type packages 11a, 11b have photodiodes 15a, 15b, respectively, die-bonded thereto by means of epoxy resin, eutectic solder or the like. Au wires 14a, 14b bonded to the photodiodes 15a, 15b, respectively, and caps 12a, 12b having light permeable windows formed of Kovar glass 13a and a sapphire plate 13b welded to the packages 11a, 11b, respectively, as shown in FIGS. 1a and 1b. These prior art packages, however, have the drawback that lack of dimensional precision as between the caps 12a, 12b and packages 11a, 11b occurring in the manufacture of the same, and also the presence of the Au wires 14a, 14b on the photodiodes made the distance between the upper surface of the photodiode 15a or 15b and Kovar glass 13a or sapphire plate 13b great and as a result, the optical coupling efficiency between the photodiode 15a or 15b and optical fiber 16a or 16b is imparied.
Next, an example of a second type of prior art package which is different from the first type with respect to the light receiving system, is shown in FIG. 2. In this example, a conventional TO-46 type package 21 is provided with a through opening 23, a photodiode 25 is die-bonded to the package 21 coaxially with the through opening 23. Au wire 24 is bonded to the photodiode 25 and a cap 22 is welded to the package 21 to provide the construction as shown in FIG. 2. This construction, however, also has the drawback that the distance between the photodiode 25 and optical fiber 26 is great resulting in the optical coupling efficiency being impaired as described in connection with the first type of the prior art.
FIG. 3 shows an example of a third type of the prior art which eliminates the drawback inherent in the second type referred to hereinabove. This third type of prior art example is different from the second type in that the optical fiber 36 is adapted to be inserted into the through opening 33 in the package 31 so that the distance between the photodiode 35 and optical fiber 36 can be shortened. In this example, however, the optical fiber 36 tends to contact the photodiode 35 and damages the photodiode 35 when the optical fiber 36 is inserted into the package 31 and as a result, it has the disadvantage that the assembly of the components thereof is very difficult.
FIG. 4 shows an example of a fourth type of the prior art in which Kovar glass 43 seals the through opening 23 in the package in the second type of prior art example shown in FIG. 2. The fourth type of prior art example, however, also has the same drawback as described in connection with the second type.
Needless to say, even when other metals or ceramics are used in the construction of the above-mentioned examples of any type of the prior art package, the same drawback is inevitably present.
Further drawbacks common to the second and fourth type of prior art examples will now be described. FIG. 5 shows the light receiving portions in the second type of prior art examples. In the construction shown in FIG. 5a, a photodiode 25 including a light receiving area 25a is die-bonded to the upper surface of a metal or ceramic substrate 21. The angular aperture for the incoming light is limited by the edge 21a of the substrate 21. FIG. 5b shows the state wherein the photodiode 25 moves out of place at the time of die-bonding and the angular aperture for the incoming light is substantially limited on one side thereof. Thus, in these examples, a rigidly precise die-bonding technology is required. In these examples also, in order to increase the above-mentioned angular aperture, it is required that the thickness of the substrate 21 is lessened and the cross-sectional area of the through opening 23 is increased. However, when the thickness of the substrate 21 is lessened, the package itself presents a problem with respect to strength and when the cross-sectional area of the through opening 23 is increased, there is the problem that the size of the photodiode 25 has to be increased accordingly. Furthermore, in these examples, there is the problem that ceramics or metal have to machined to allow formation of a through opening 23 by a mechanical process, the precision of which imposes limitations on the construction of the package.